I came to this board because I was self-medicating this way for the first time since I'd gotten married - after over 20 years of fidelity. None of my MDs have diagnosed me with bipolar, and my newest psychiatrist says he feels I just have dysthymia and situational depression, but that what I'm doing is getting a dopamine high as a coping mechanism. So I asked for a drug with dopamine, and he said the only one with any dopamine activity was Wellbutrin. I don't know if that is helpful for people who are bipolar or not. The medicine he started me on was Remeron, and I am hopeful it is going to get me back on track. I hope you see an MD who can help you. I'm not at a point where I can really feel guilt about it. It has just felt like survival.
Yes its the loss of sex control that is part of the bipolar and especially the mania. Marital infidelity is not really uncommon in today's society. Neither is sexual promiscuity. I don't tend to pass judgement as long as its consenting adult and safe sex. However, any form of losing control is not good and if its part of the general pattern of mania probably included other forms of losing control that you may not have thought about, speeded up activity (technically psychomoter agitation) racing thoughts, an inability to concentrate. But you wouldn't blame yourself for those things. So don't blame yourself for this. And I would think that some aspect of the relationship was part of it so the best thing is marriage counseling. You could both discuss the issue there.
Sexual Promiscuity is a bipolar behaviour, along with risk taking, gambling, binge spending and a host of other things.
Have you spoken to your therapist about this? Its an issue that responds well to therapy I think, its not one of my behaviours but I have others.
As for forgiveness the first step is acceptance and stopping blame - too often we blame ourselves for things outside of our control and we spend a lot of time beating ourselves up over things we cannot prevent.
I think the first thing you need to do is take a deep breath and accept that this happened and you need help and then ask for it. As to telling your spouse, its going to be a time thin - until you understand yourself this is something which you need to place to the side a bit - telling your spouse now at this stage would I suspect damage or destroy a relationship when you are at a fragile place.
Definetly a therapist here, if you do not have one get one, they are invaluable resources.